Digestive System Overview

Outcomes of Digestive System Study

• Define digestion and absorption:
  • Digestion: The process of breaking down food into smaller components that can be absorbed by the body.
  • Absorption: The process of taking nutrients from the digestive system into the bloodstream.
• List the functions of the GI tract:
  • Ingestion of food
  • Digestion (mechanical and chemical)
  • Absorption of nutrients
  • Elimination of waste
• Identify the layers of the GI tract:
  • Mucosa: Innermost layer that comes in contact with food.
  • Submucosa: Contains blood vessels, nerves, and connective tissue.
  • Muscularis: Responsible for peristalsis and segmentation.
  • Serosa: Outermost protective layer.
• Identify components of the peritoneum:
  • Greater omentum: Fatty layer draping over intestines.
  • Mesentery: Fold of peritoneum attaching intestines to the abdominal wall.
• Explain the composition of saliva:
  • Composed of water, electrolytes, mucus, enzymes (amylase, lipase).
  • Saliva is produced by salivary glands (parotid, submandibular, sublingual).

Stomach Anatomy and Function

• Stomach size when empty: about the size of a large sausage.
• Folds in the stomach are called rugae; these allow for expansion and mixing of food.
• Exit from the stomach is via the pyloric valve.
• Functions of the stomach:
  • Acts as a mixing and holding area for food.
  • Begins protein digestion.
  • Converts food bolus into a liquid forms called chyme.
  • Absorbs substances such as glucose, water, ions, and alcohol.

Gastric Juice Composition

• Contains HCl:
  • Destroys microbes in food.
  • Converts pepsinogen into pepsin for protein digestion.
• Pepsin breaks down some proteins into peptides.
• Mucous protects the stomach wall from being digested by its own acid.

Pancreatic Juice Composition

• pH of about 7.1 to 8.2 (alkaline) due to sodium bicarbonate and digestive enzymes.
• Pancreatic amylase: Digests starch into maltose.
• Pancreatic lipase: Breaks down triglycerides into monoglycerides and fatty acids.
• Trypsin: Digests proteins to peptides.

Liver Functions

• Bile production: Stored in the gallbladder; emulsifies fats.
• Phagocytosis: Destroys red blood cells and bacteria.
• Nutrient processing: Manages carbohydrates, fats, and proteins.
• Storage: Holds iron, glycogen, and vitamins.
• Detoxification: Removes toxins from alcohol and drugs.
• Heat production: Maintains body temperature.

Bile Composition

• Transported via a duct system to the gallbladder for storage.
• Components include:
  • Water
  • Bicarbonate ions (HCO3-): Neutralizes acidic chyme.
  • Bile salts: Formed from cholesterol, important for fat emulsification.
  • Bile pigments: Such as bilirubin, which are waste products.

Small Intestine

• Main site of digestion and absorption, divided into:
  • Duodenum (25 cm)
  • Jejunum (1 m)
  • Ileum (2 m)
• Diameter: 2.5 cm.
• Total length: approximately 3.25 m.

Modifications for Absorption

1. Circular folds: Increases surface area and slows chyme movement for better absorption.
2. Villi: Finger-like projections with blood capillaries and lacteals for fat absorption.
3. Microvilli: Tiny projections on villi that further increase surface area.
4. Length: Long length increases absorption efficiency.

Nutrient Absorption

• Carbohydrates: Absorbed as monosaccharides.
• Proteins: Absorbed mainly as amino acids.
• Vitamins: B and C absorbed into blood; lipids, D, E, K, A absorbed into lacteals.

Large Intestine Anatomy and Function

• Extends from the ileocecal sphincter to the anus.
• Regions include:
  • Cecum
  • Colon: Ascending, transverse, descending, sigmoid.
  • Rectum and Anus.
• Diameter: 6.5 cm, Total length: 1.5 m.

Chemical Digestion in the Large Intestine

• No enzymes secreted, only mucus.
• Bacteria ferment undigested carbohydrates into gases.
• Bacteria produce Vitamins K and B.

Absorption of Water

• 9 liters of fluid secreted into GI tract each day.
• Small intestine reabsorbs 8 liters; large intestine absorbs 90% of the remaining liter, primarily by osmosis into capillaries in villi.